1. Field of the Invention
The present invention relates to a polyvinyl alcohol (hereinafter referred to as "PVA")-based synthetic fiber useful for industrial materials for which hot water resistance is required, in particular for fiber-reinforced cement (hereinafter referred to as "FRC") which is subjected to autoclave curing, and a process for its production.
The present invention also relates to an FRC reinforced with the above PVA-based synthetic fiber and having excellent dimensional stability, in particular excellent toughness under wet conditions.
2. Description of the Prior Art
Health injury caused by asbestos has become apparent in recent years, and its use is becoming more and more legally restricted. PVA-based synthetic fiber has highest strength and modulus among general-purpose fibers and also high adhesiveness with cement and resistance to alkali. Demand for the fiber as a replacement of asbestos in the field of FRC is therefore rapidly growing.
PVA-based synthetic fiber is, however, inherently poor in wet heat resistance and dissolves at a wet temperature of at least about 130.degree. C., whereby its autoclave curing is impossible and only room-temperature curing has been used. Although carbon fiber is used as an asbestos replacement in some uses at present, carbon fiber has poor adhesiveness with cement matrix and thus produces only poor reinforcement effect. Moreover, carbon fiber is far more expensive than asbestos or PVA-based synthetic fiber.
Attempts have been made to improve the wet heat resistance of PVA-based synthetic fiber. For example, Japanese Patent Application Laid-open No. 133605/1990 discloses a process which comprises blending an acrylic polymer, or crosslinking the fiber surface with an organic peroxide, isocyanate, blocked isocyanate, urethane-based compound, epoxy-based compound or the like.
However, blending of an acrylic polymer may not be successful, since the acrylic polymer blended will dissolves out during solvent extraction process in the spinning of the blend. Even if part of undissolved acrylic polymer crosslinks, the crosslinkage that is formed by ester bond readily hydrolyzes with the alkali of cement, thus being unable to withstand autoclave curing.
Besides, crosslinking of only fiber surface results, during autoclave curing, in swelling and dissolution from inside of the fiber, whereby satisfactory wet heat resistance cannot be obtained.
The concept of surface crosslinking is to restrict the regions crosslinked to only the fiber surface, because crosslinked structure inside the fiber will hinder high-draft drawing of the fiber so that high-strength fiber becomes difficult to obtain. However, since the PVA fiber obtained under this concept is crosslinked preferentially on its surface, the fiber swells or dissolves from its inside when contacted with hot water, as described above.
This phenomenon is more marked when the fiber is used for FRC. That is, reinforcement fiber for FRC is generally mixed into cement in the form of short cut fibers, the cut surfaces of which are directly exposed to vapor and cement components containing alkali. Then, central part of the cross-sections which is not crosslinked swells or dissolves. Accordingly, crosslinking of fiber surface only cannot improve the wet heat resistance applicable to FRC. The present inventors have actually confirmed that, with the crosslinked fiber of this type, reinforcement effect diminishes during autoclave curing at 140.degree. C.
Japanese Patent Application Laid-open No. 249705/1990 discloses a process for improving the fatigue resistance of a PVA fiber used for tire cords, which comprises crosslinking the fiber. To achieve the crosslinking, the disclosure includes, in addition to a process which comprises treating a PVA fiber cord with a crosslinking agent, a process which comprises adding a crosslinking agent to a spinning dope solution or a coagulating bath so that the agent can penetrate into the inside of the fiber and crosslinks there. However, if a crosslinking agent is added to a spinning dope solution, it will dissolve out into the coagulating bath used. If a crosslinking agent is added to a coagulating bath, it cannot penetrate into and crosslink the inside of the resulting fiber, since the coagulating bath does not diffuse there but simply acts to remove the solvent used from the extruded streams of the spinning dope solution used. In both cases, the improvement of the wet heat resistance, which is an object of the present invention, is not achieved.
Japanese Patent Application Laid-open No. 120107/1988 discloses a process which comprises formalizing to a degree of formalization of 5 to 15 mol% a PVA-based synthetic fiber having been drawn in a drawing ratio of at least 15. This level of formalization, however, renders hydrophobic only very small part of the amorphous region of the fiber so that the finished fiber cannot withstand autoclave curing. As described in detail later herein, such a fiber has a gel elasticity as defined in the present invention of 1-2.times.10.sup.-3 g/cm.multidot.d at most and is thus clearly distinguished from the fiber of the present invention.
By the way, autoclave curing as so far discussed is conducted to secure a good dimensional stability of cement products. During the curing, calcium oxide and silica react to form a crystal called tobermolite. This reaction proceeds under a wet heat condition of at least 140.degree. C. preferably at least 160.degree. C. which can relatively shorten the curing time.
While the temperature desired for practical curing thus at least 160.degree. C., it has been impossible, as described above, with conventional techniques to produce a PVA-based synthetic fiber that can withstand such a severe curing condition stably.
Autoclave curing generally improves dimensional stability but decreases bending strength and strain, i.e. toughness of bending, especially under wet conditions. Reinforcing fibers to be autoclave-cured are therefore required to exhibit the effect of improving the bending toughness. A toughness ratio under wet condition of at least 1.2 is desirable for practical purposes.
Although carbon fiber is, as described above, in some cases used as an asbestos replacement that can withstand such hard treatment as autoclave curing, the fiber can hardly improve bending toughness due to its low elongation. This is another reason, i.e. besides its very high price as compared with asbestos or PVA-based synthetic fiber, why carbon fiber has not been widely used.
Japanese Patent Application Laid-open No. 213510/1991 discloses an autoclave-curable PVA-based synthetic fiber having a "hot water resistance" of at least 140.degree. C. The specification mentions in its Example one having a hot water resistance at 158.degree. C. The hot water resistance as referred to in that specification is, however, the temperature of water in which a fiber is dissolvable. The fiber disclosed therefore cannot withstand the autoclave curing discussed herein.